The present invention relates to an offset adjusting feedback system for adjusting offset in an analog signal derived from a digital signal. A typical use would be in digitally based communication channel receivers or monitors.
Communication channels can be entirely analog extending from the information source to the transmitter, and then along the channel link to the receiver, and finally to the user of the transmitted information. Communication channels can also be digitally based, either totally or partially. A typical situation is to have a digital information source but where the information is sent in analog form over an analog channel link to a receiver which reconverts the information to a digital format.
Among the types of signal degradation which can be encountered in a digitally based communications channel is the incorporation in the analog signal in the channel of a constant or nearly constant additive signal termed an offset. For example, in a three-level partial response digitally based communications channel, the incoming baseband analog signal to the receiver detectors may not be symmetrical about points in this signal intended to equal the center expected incoming signal level, usually as a result of distortion in the generating or processing of signals in the digitally based communications channel. The result is that there appears to be an offset between the center expected incoming signal level and the points in the incoming baseband analog signal intended to equal the center expected incoming signal level. For proper monitor system performance, the points in the incoming baseband analog signal which are intended to be at the center expected incoming signal level should be maintained there for at least two reasons.
First, the phasing of the data samples sought to be corrected in a phase correction feedback loop will not fully correct if the intended center expected incoming signal level points in the incoming baseband analog signal do not occur at the center expected incoming signal level. And secondly, values obtained from the incoming baseband analog signal interpreted as being the intended upper and lower expected incoming signal levels will not reflect the actual lower and upper expected incoming signal levels being measured with respect to the center expected incoming signal level if the intended center expected incoming signal level points in the incoming baseband analog signal are not at the center expected signal level.